Digital photography has overtaken traditional film based photography as the routine means by which images are taken and stored. Initially confined to single purpose camera devices, over time digital cameras incorporated into cellular phones have grown increasingly popular, and at the present time, the majority of digital images captured by individuals for non-commercial purposes are captured using a digital camera incorporated within a cellular phone. Accordingly, as the desire for cellular phones capable of acting as digital cameras has grown, the desire for the digital images captured by those cellular phones to be of higher quality has correspondingly grown.
For years, the image sensors used within the digital camera systems of cellular phones increased in resolution (i.e., pixel count), with top of the line cellular phones incorporating image sensors having more than 16 million pixels (MP), and in one case, over 40 MP.
This increase in resolution was accomplished in part by shrinking the size of individual pixels. This in turn results in each pixel having a reduced charge storage capacity, which means that each pixel captures less light. Since the maximum signal to noise ratio is a function of the square root of the charge storage capacity, these smaller pixels ultimately result in a worse signal to noise ratio.
Consequently, the trend to greater numbers of smaller pixels began to reverse, and the current trend is toward smaller numbers of larger pixels. Since image sensors for cellular phones are desired to be small and compact, the challenge is therefore to design sensors with larger pixels, yet keep the sensor size as compact as possible.
Due to this challenge, rolling blade operated type pixels are commonly employed instead of global shutter operated pixels, due to the traditionally smaller area occupied by rolling blade shutter pixels. However, as will be explained, rolling blade shutter pixels have drawbacks compared to global shutter pixels.
In a rolling blade shutter, an array of pixels are processed line by line, with one being integrated and another being read out for each movement of the shutter. The shutter moves over the array so that the pixels are exposed for the same amount of time, but not at the same time. A rolling blade shutter may not work well when taking images of fast moving objects, such as fans, helicopter blades, or propellers.
With a global shutter, the pixels of the array are simultaneously released from reset and start to integrate simultaneously. As a result, the drawbacks of a rolling blade shutter are not present. After a specific period, the pixels are then read out simultaneously into a temporary storage, which may be located inside the pixel. This temporary storage is then scanned out row by row where the signal is amplified or converted into a digital value.
Since the pixels integrate simultaneously, each pixel has at least one dedicated storage capacitor. The various challenges involved in the design of these storage capacitors result in the consumption of an undesirable amount of surface area. So as to make global shutters more size competitive with rolling blade shutters, new designs are desired.